1. Field of the Invention
This invention is concerned with double layer capacitors and, more particularly, with an improved method for forming the electrodes of such double layer capacitors.
2. Description of the Prior Art
Double layer capacitors are disclosed in prior U.S. Pat. Nos. including: 3,536,963, 3,652,902, 4,327,400, 4,394,713.
In general, the above patents describe double layer capacitors which comprise a pair of polarization electrodes having a separating medium therebetween. The electrodes are composed of a solid and liquid phase and the electric double layer which characterizes these capacitors is formed at the interface between the solid and liquid (electrolyte) phases of the electrodes. The separating medium acts as an electronic insulator between the electrodes, but is sufficiently porous to permit ion migration therethrough.
A double layer capacitor is therefore made up of two half-cells assembled on either side of a separator element. Each half-cell comprises an electrode element, a horizontally enclosing cell wall for the electrode element and a conductor element contacting one end wall of the electrode element. The cell wall and conductor element, when assembled, thus provide a structure having an electrode cavity in which the electrode is formed.
Double layer capacitors can be made in miniature size, yet they exhibit very large capacitance when compared with conventional capacitors of similar or near similar size.
In the present construction of the double layer capacitor, the electrode is made up of high surface area carbon particles associated with an electrolyte such as H.sub.2 SO.sub.4, at a concentration of 25 wt. % H.sub.2 SO.sub.4. The electrode material also contains about 0.2 moles of H Br per liter which acts as a voltage regulator. When ready for deposit in the electrode cavity, this electrode material has the consistency of a damp powder; i.e., the powder particles do not flow readily and tend to pack down and adhere to each other under even light pressure. The electrode is made by first preparing carbon electrode material as follows:
A. A quantity of high surface area carbon, which may be an activated carbon, is given four acid washes with an excess amount of acid; an exemplary acid being a 25% solution of H.sub.2 SO.sub.4. PA0 B. The carbon-acid mix is filtered and, following filtration, the acid-saturated carbon is pressed at 3000 psi to force out the excess acid, leaving a damp powder product. PA0 C. The damp powder is then weighed out in an amount adequate to fill the cell cavity and then is pressed into the cavity.
This method of preparing electrodes for double layer capacitors may be termed the "damp powder" method.
It will be appreciated that this damp carbon electrode material is difficult to meter accurately, does not invariably completely fill the cell cavity when pressed into it, and even when it does, the density of electrode material may vary from point to point within the electrode adversely affecting the electrical characteristics of the finished capacitor.
A more fluid electrode material could be more accurately metered and would minimize or eliminate the problems of filling the cell cavity and obtaining uniform density. However, previous attempts to use a more fluid electrode material failed; either the electrical resistance of the capacitor was too high or the electrode material exuded from the cell under the pressure exerted. Substantial improvement in electrical properties of double layer capacitors can be attained by a method for forming electrodes which avoids the disadvantages of the present method outlined above.